We report a significant poleward surge in thermospheric winds at subauroral and midlatitudes following the 17–18 March 2015 great geomagnetic storm. This premidnight surge is preceded by strong westward winds. These disturbances were observed over three sites with geodetic latitudes 35–42°N in the American sector by Fabry‐Perot interferometers at 630 nm wavelength. Prior to the wind disturbances, subauroral polarization streams (SAPS) were measured by the Millstone Hill incoherent scatter radar between 20 and 02 UT. We identify the observed neutral wind variations as driven by SAPS, through a scenario where strong ion flows cause a westward neutral wind, subsequently establishing a poleward wind surge due to the poleward Coriolis force on that westward wind. These regional disturbances appear to have prevented the well‐known storm time equatorward wind surge from propagating into low latitudes, with the consequence that the classic disturbance dynamo mechanism failed to occur.

We use incoherent scatter radar observations at Millstone Hill (MHO) and Arecibo (AO) and topside ionosphere in situ Defense Meteorological Satellite Program (DMSP) observations during the great geomagnetic storm on 17–18 March 2015 to conduct a focused study on ion‐neutral coupling and storm time ionosphere and thermosphere dynamics. Some of these observations were made around the time of large ionospheric drifts within a subauroral polarization stream (SAPS). During the storm main phase, we identify multiple disturbance characteristics in the North American late afternoon and dusk sector. (1) Strong subauroral westward drifts occurred between 20 and 24 UT near MHO, accompanied by a storm enhanced density plume passage over MHO in the afternoon with a poleward/upward ion drift. The strongly westward flow reached 2000 m/s speed near the poleward plume edge. (2) Prompt penetration electric field signatures, appearing as poleward/upward ion drifts on the dayside over both MHO and AO, were consistent with DMSP vertical drift data and contributed to plume development. (3) Meridional wind equatorward surges occurred during daytime hours at MHO, followed by 2–3 h period oscillations at both MHO and AO. The zonal electric field at AO was strongly correlated with the wind oscillation. (4) Large ion temperature enhancements as well as 50+ m/s upward ion drifts throughout the E and F regions were observed during the SAPS period. These were presumably caused by strong frictional heating due to large plasma drifts. The heating effects appeared to drive significant atmospheric upwelling, and corresponding ion upflow was also observed briefly. This study highlights some of the important effects of fast plasma transport as well as other disturbance dynamics on ion‐neutral coupling during a single intensification period within a great geomagnetic storm.

[1] The incoherent scatter radar mode and data extraction method used in topside experiments at the Arecibo Observatory are discussed. Helium ion concentrations in the lower topside ionosphere over Arecibo are presented for low and high solar flux periods of the same season (October). Data from 8

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High resolution measurements of the geocoronal Balmer alpha (Hα) emission were made with a Fabry‐Perot interferometer at Arecibo, Puerto Rico, in six observing campaigns between March 1980 and November 1984. Four of these included simultaneous radar measurements of the topside F region. The nighttime variation of the zenith Hα intensity is highly symmetric about solar midnight. However, effective hydrogen temperatures near the morning exobase are generally hotter than near the evening exobase, implying a depletion of exobase hydrogen density in the early morning hours. The exospheric temperature is derived from the Doppler line profiles and is compared to the topside F region O+ temperature. The cooling of H relative to O+ due to the escape of hot, energetic H is not obvious in our data, and it appears that a downward flux of hot protons often causes heating of the bound neutral H population by charge exchange during low to moderate solar activity conditions. This downward flux is usually strongest between 0200 and 0600 LT, and occurs throughout the night in the late fall and early winter. We have detected a deficiency of inbound high‐energy hydrogen relative to the high‐energy outbound population in radial velocity Hα profiles. We have also detected large distortions of the Hα profile wings during low to moderate solar activity periods and during periods of large downward ion speeds. We attribute these wing distortions to efficient charge exchange of H with H+. A narrowing of zenith profile widths near midnight demonstrates the gravitational cooling of H due to the inverse relationship between geocentric distance and the kinetic energy required for escape. We have also searched for evidence that satellite hydrogen populations with small orbital eccentricities may be contributing to narrow line widths.